Chemically induced stimulation of subterranean carbonaceous formations with gaseous oxidants

ABSTRACT

A method for increasing the methane recovery from a subterranean carbonaceous formation by chemically stimulating the formation of cleats and increasing the surface area in the carbonaceous formation by injecting a gaseous oxidant into the carbonaceous formation to stimulate the formation of cleats in and increase the surface area of carbonaceous material in the carbonaceous formation; and thereafter producing methane from the carbonaceous formation at an increased rate. The gaseous oxidant may be ozone, oxygen or combinations thereof.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 08/846,994,now U.S. Pat. No. 5,865,248, entitled "Chemically Induced PermeabilityEnhancement of Subterranean Coal Formation" filed Apr. 30, 1997 byWalter C. Riese and Stephen V. Bross which is a continuation-in-part ofU.S. Ser. No. 08/594,725, now U.S. Pat. No. 5,669,444, entitled"Chemically Induced Stimulation of Coal Cleat Formation" filed Jan. 31,1996 by Walter C. Riese and Stephen V. Bross.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for increasing the rate of productionof methane from a subterranean carbonaceous formation by chemicallystimulating the formation with a gaseous oxidant to increase theproduction rate of methane from the formation. The invention isapplicable to the enhanced recovery of methane from formationsconsisting of carbonaceous materials deposited with inorganic materials,such as occur in carbonaceous shale formations. The increased productionrate is accomplished by increasing the surface areas of the containedorganic material fragments, which contain hydrocarbons, by inducing theformation of cleats and other new surfaces in these carbonaceousmaterials, thereby facilitating the desorbtion of light hydrocarbonsfrom these carbonaceous formations. Carbonaceous formations such asshales, are composed in part of clay minerals. The invention is alsoapplicable to the enhanced recovery of light hydrocarbons which areadsorbed to these clay minerals.

2. Brief Description of the Prior Art

Substantial quantities of methane gas are found in subterranean coalformations and in other formations containing carbonaceous materials,which may include macerals, kerogens, and other organic materials andwhich are present with inorganic materials such as sands, clays and likeclastic materials in the formation. Such other formations are referredto herein as "carbonaceous formations".

A variety of processes have been used in attempts to recover the methanefrom such formations, especially coal formations, more efficiently.

The simplest process is the pressure reduction process wherein aborehole is drilled into a coal formation from the surface and methaneis withdrawn from the borehole by reducing the pressure to cause methaneto be desorbed from and flow from the coal formation into the boreholeand to the surface. This method is not efficient because coal formationsare generally not extremely porous and the majority of the methane isgenerally not found in the pores of the coal formation but is absorbedin or adsorbed to the coal. While methane can be produced from coalformations by this process, the production of methane is relativelyslow.

In some coal formations, the natural permeability is sufficient to allowthe removal of in situ water to permit the enhanced recovery of methane.In such formations, cleat systems developed during the coal beddiagenesis and burial history provide channel ways through which waterand methane migrate to the production wells for removal. This removal ofwater or "de-watering" of the coal formations removes water from thechannel ways and permits the flow of methane through the channel waysand to a production well at a greater rate.

Many coal formations do not have extensively developed cleat systems orhave cleat systems which are not fully developed. These coal formationshave very low permeability to water and gas and do not yield water orgas at significant rates. As a result, the water fills the cleats, andthe recovery of methane from such coal formations is difficult orimpossible at significant rates. Such low permeability water-containingcoal formations may be either water saturated or less than fully watersaturated. It appears that coal formations with better developed cleatsystems may have been exposed to a diffusive oxidizing fluid of sometype during the geologic past whereas coal formations with lessdeveloped cleat systems do not show evidence of past exposure to anoxidizing fluid.

Many formations containing carbonaceous materials in combination withinorganic materials show similar behavior. These are referred to ascarbonaceous formations. Many such formations contain large quantitiesof methane, or other absorbed or adsorbed light hydrocarbons such asmethane, but the methane is not readily recovered from such formationsbecause the permeability and exposed surface area of the containedcarbonaceous materials are too low to permit the efficient release ofmethane from the formation.

The terms "absorbed" and "adsorbed" are used interchangeably in thediscussion herein to refer to methane or other light hydrocarbons whichare retained in or on the surfaces of the carbonaceous materials and themethane or other light hydrocarbons which are retained in or on thesurfaces of the clay-mineral materials which are present in thecarbonaceous formations.

Accordingly, continuing efforts have been directed to the development ofmethods for replicating the effects of the conditions which formed thebetter developed cleat systems in coal formations and increasing theproduction rate of methane from carbonaceous formations.

SUMMARY OF THE INVENTION

According to the present invention, the rate of recovery of methane froma subterranean carbonaceous formation is increased by positioning atleast one well from the surface into the formation; injecting a gaseousoxidant into the formation; maintaining the gaseous oxidant in thecarbonaceous formation for a selected time to stimulate the formation ofadditional surface area or cleats in the organic materials contained inthe formation; and producing methane from the formation at an increasedrate. The injection of the gaseous oxidant into the formation, and themaintenance of the gaseous oxidant in the formation for a selectedperiod of time stimulates and facilitates the desorbtion of methane andother light hydrocarbons from the carbonaceous materials and theclay-mineral constituents of the formation; allows the methane tomigrate from the formation into the wellbore; and allows the methane tobe produced from the formation at an increased rate.

Some suitable oxidants are ozone, oxygen, and combinations thereof.

The rate of production of methane from a subterranean carbonaceousformation penetrated by at least one injection well and at least oneproduction well is increased by:

a) Injecting a gaseous oxidant into the formation through the injectionwell; and

b) Producing methane from the formation through the production well atan increased rate.

The present invention is effective to enhance methane recovery fromcarbonaceous materials disposed with inorganic materials and enhancesthe recovery of methane from the inorganic materials to which and inwhich it is adsorbed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a well penetrating a subterraneancarbonaceous formation from the surface.

FIG. 2 is a schematic diagram of a well penetrating a subterraneancarbonaceous formation from the surface wherein the carbonaceousformation has been fractured.

FIG. 3 is a schematic diagram of an injection well and production wellpenetrating a subterranean carbonaceous formation from the surface.

FIG. 4 is a schematic diagram of an injection well and a production wellpenetrating a subterranean carbonaceous formation from the surfacewherein the carbonaceous formation has been fractured from the injectionwell.

FIG. 5 is a schematic layout of a 5-spot injection and production wellpattern.

DESCRIPTION OF PREFERRED EMBODIMENT

In the discussion of the Figures, the same numbers will be usedthroughout the specification to refer to the same or similar components.

In FIG. 1, a carbonaceous formation 10 penetrated from a surface 12 by awellbore 14 is shown. The wellbore 14 includes a casing 16 positioned inthe wellbore 14 by cement 18. While wellbore 14 is shown as a casedwellbore it should be understood that in the preferred embodiments shownin the Figures, cased or uncased wellbores could be used. Alternatively,the casing 16 could be extended into or through carbonaceous formation10 with perforations through the casing in the carbonaceous formation 10providing fluid communication between carbonaceous formation 10 andwellbore 14. Wellbore 14 extends into carbonaceous formation 10 andincludes a tubing 20 and a packer 22. Packer 22 is positioned to preventflow between the outer diameter of tubing 20 and the inner diameter ofcasing 16. Wellbore 14 also includes equipment 24 adapted to inject agaseous or liquid stream into carbonaceous formation 10 or to recover agaseous or liquid stream from carbonaceous formation 10.

In the practice of the present invention, a gaseous oxidant is injectedas shown by an arrow 26 through tubing 20 into carbonaceous formation 10as shown by arrows 28. The zones treated are shown by circles 30. Thegaseous oxidant is injected into carbonaceous formation 10 for aselected time to enhance or stimulate the formation of additionalsurface area or cleats in the organic materials contained incarbonaceous formation 10. The gaseous oxidant is injected for a periodof time and in a quantity considered sufficient to increase the abilityof the organic materials present in carbonaceous formation 10 in thezones 30 to desorb the methane and other light hydrocarbons which areabsorbed on and in the organic materials. After a selected period orafter a selected amount of the gaseous oxidant has been injected, thewell is shut in for a period of time which may be up to or greater than24 hours. Typically, the well is shut-in until the pressure in thewellbore returns to the formation pressure and thereafter for at least12 additional hours. Alternatively, a sufficient period of oxidantpresence in carbonaceous formation 10 may have elapsed during theinjection of the gaseous oxidant. The shut-in period allows formigration of the gaseous oxidant into carbonaceous formation 10 tooxidize components of carbonaceous formation 10; thereby increasing thesurface area of, and cleats in, the organic materials present incarbonaceous formation 10. The shut-in period also allows for migrationof the oxidant into carbonaceous formation 10 to separate methane andother light hydrocarbons which are adsorbed to the clay-minerals presentin carbonaceous formation 10. Subsequent to the shut-in period, water,methane or both may be recovered from carbonaceous formation 10 tode-water carbonaceous formation 10 in the zones 30 and produce methane.The term "de-water" as used herein does not refer to the completeremoval of water from carbonaceous formation 10, but rather to theremoval of sufficient water from carbonaceous formation 10 to openpassage ways in carbonaceous formation 10 so that methane can beproduced through the passage ways from carbonaceous formation 10.

The gaseous oxidant contains an oxidant selected from the groupconsisting of ozone, oxygen, and combinations thereof. Of these, ozoneis preferred. When ozone is used the concentrations of ozone in thegaseous oxidant may be up to 100%. Any suitable gaseous diluent may beused if concentrations of ozone below 100% are used.

When oxygen is used the concentrations are suitably up to about 50percent volume percent of the gaseous oxidant mixture withconcentrations up to about 30 volume percent being preferred and withconcentrations from about 23 to about 35 volume percent being desirable.The oxygen-containing gaseous oxidant mixture may be air, but ispreferably oxygen enriched air containing oxygen at the concentrationsstated above. The oxidants can be used in gaseous oxidant mixtures incombination within the ranges discussed above.

Desirably the oxidants are used in gaseous oxidant mixtures which may beadjusted to avoid combustion in the wellbore or coal seam, to avoidgasification or liquification of carbonaceous materials near thewellbore, and the like. Applicants seek to physically modify thestructure of the carbonaceous formation to stimulate the formation ofcleats and increase the surface area of the carbonaceous materials inthe carbonaceous formation in order to increase the permeability of theformation to gas and liquids while avoiding combustion processes.Application of the gaseous oxidant mixture to the carbonaceous formationsurfaces, which may be accessed via naturally occurring fractures,artificially created fractures, other existing passageways in thecarbonaceous formation and the like, provides access to the carbonaceousmaterials, which may include coal macerals, to affect the maceralcomposition, maceral architecture and bonding between maceral faces,thereby stimulating the formation of cleats and a cleat system andincreasing the surface area of the carbonaceous materials and increasingthe permeability of the carbonaceous formation. This treatment does notresult in the removal of solid or particulate carbonaceous material fromthe carbonaceous formation, or combustion of the carbonaceous material.Rather, the structure of the carbonaceous material is modified; bycreation of cleats and cleat systems, thus increasing the surface areaand increasing the permeability in the carbonaceous formation to achievethese objectives without removal of solid or particulate carbonaceousmaterial from the formation and without gasification or other physicaldestruction of the solid carbonaceous material in the carbonaceousformation.

The injection of the gaseous oxidant facilitates the formation ofadditional free surface area and cleats in the carbonaceous formationand facilitates the release of methane and other light hydrocarbons fromthe organic materials and from the surfaces of the clay-minerals towhich they are adsorbed.

In the embodiment shown in FIG. 1, a single well is used for injectionof the gaseous oxidant to chemically enhance or stimulate the formationof additional free surface area and cleats in the organic materialspresent in carbonaceous formation 10 and facilitate the release ofhydrocarbons adsorbed on clay-minerals present in zones 30, to result inthe release of formation water and an increase in the methane productionrate from carbonaceous formation 10. The term "increase" as used hereinrefers to a change relative to the untreated carbonaceous formation.

In FIG. 2, a similar embodiment is shown except that carbonaceousformation 10 has been fractured by fractures 32. The operation of thewell is basically the same as that shown in FIG. 1 except thatcarbonaceous formation 10 has previously been fractured. For instance,it may be desirable to use a conventional fracturing application, ifcarbonaceous formation 10 is sufficiently impermeable, as an initialstimulation method followed by the gaseous oxidant. In such instances,the well is desirably shut-in as discussed previously and the oxidantsare selected from the same oxidant materials discussed previously. Thefractures are formed in carbonaceous formation 10 prior to injection ofthe gaseous oxidant. The gaseous oxidant could be injected above orbelow the fracture gradient (pressure) if desired.

In FIG. 3, an injection well 34 and a production well 36 penetratecarbonaceous formation 10 from surface 12. Injection well 34 is spacedapart from production well 36 at a spacing based upon thecharacteristics of the particular carbonaceous formation and the like.According to the present invention, the gaseous oxidant is injected intocarbonaceous formation 10 through injection well 34 as shown by arrow 26and arrows 28 to treat zones 30 which may extend from injection well 34in a generally circumferential direction, but generally extendpreferentially toward a nearby production well or production wells.Production well 36 is positioned to withdraw water and methane fromcarbonaceous formation 10. The production of water and methane throughproduction well 36 causes the gaseous oxidant to migrate towardproduction well 36. Desirably, injection of the gaseous oxidant iscontinued until an increased water volume is detected in production well36 or until a desired increase in permeability or surface area or anincrease in the volume of fluids produced is achieved. The increase inpermeability, surface area or volume of fluids produced from productionwell 36 is indicative of increased permeability, surface area or both incarbonaceous formation 10 and is attended by the release of additionalquantities of fluids from carbonaceous formation 10 for production asshown by arrows 38 through production well 36 and by an arrow 40. Arrows38 are shown directed toward production well 36 from both directions incontemplation that water will continue to be recovered at a lower ratefrom untreated portions of carbonaceous formation 10.

The embodiment shown in FIG. 4 is similar to that shown in FIG. 3 exceptthat carbonaceous formation 10 has been fractured by fractures 32.Fractures 32 in the embodiment shown in FIG. 2 can be of substantiallyany extent. By contrast, in the embodiment shown in FIG. 4, fractures 32desirably extend no more than half way to production well 36. Clearly,if fractures 32 extend completely into production well 36, it will bedifficult to use any kind of fluid or gas drive between injection well34 and production well 36. Desirably, the fractures extend no more thanhalf the distance between injection well 34 and production well 36. Theuse of the gaseous oxidant with fractures 32 is as discussed previously.

In FIG. 5, a 5-spot well arrangement is shown. Multiple wellarrangements, such as 5-spot well arrangements, are useful in thepractice of the present invention and may be used in a recurring patternover a wide area. Such arrangements are well known to those skilled inthe art and will be discussed only briefly. In the arrangement shown inFIG. 5, the gaseous oxidant is injected through injection well 34 totreat zones 30 to enhance the recovery of water and methane from theproduction wells 36. When the desired cleat formation or permeabilityincrease has been achieved as evidenced by the production of fluids atan increased rate from production well 36, the injection of the gaseousoxidant is stopped and injection well 34 can be converted to aproduction well. The area would then be produced through the originalproduction wells and the converted injection well. The areas of zones 30which have been treated will yield additional methane production ratesand additional ultimate methane recovery.

The method of the present invention is also useful as a pre-treatmentfor gas injection treatments to enhance the recovery of methane fromcarbonaceous formation 10. The use of carbon dioxide, either alone orwith other gases, to increase the production of methane from coalformations is well known. Similarly, the use of inert gases, such asnitrogen, argon and the like, to remove additional quantities of methanefrom the coal formations by increasing the pressure in the coalformation and thereby removing additional methane as the methane partialpressure in the atmosphere of the coal seam is decreased is well knownto those skilled in the art. The use of such processes requires that theformation be permeable to gas flow into or through the formation so thatthe methane can be recovered, and also requires that the volumes ofmethane contained in the organic materials have available free surfacesthrough which to desorb. The method of the present invention enhancesthe formation of free surfaces and cleats in the organic materials, andenhances the permeability of the carbonaceous formation where theorganic materials are more abundant and forms continuous networksamenable to treatment, and may be used prior to the use of gas sweep orgas desorption treatments to enhance the recovery of methane.

While Applicants do not wish to be bound by any particular theory, themethod of the present invention may function by creating free surfacesor a cleat system in the zones of carbonaceous formations contacted bythe gaseous oxidant. Generally the method of the present invention iseffective to increase the surface area available for the desorption ofmethane from the macerals, kerogens and other inorganic materialspresent in the formation which contain quantities of methane. It appearsthat methane may also be adsorbed to inorganic materials, particularlyclays, as well as organic materials in such carbonaceous formations, andthat the rate of methane production from both organic and inorganicmaterials is enhanced by the method of the present invention.

Having thus described the present invention by reference to certain ofits preferred embodiments, it is noted that the embodiments discussedare illustrative rather than limiting in nature and that many variationsand modifications are possible within the scope of the presentinvention. Many such variations and modifications may be consideredobvious and desirable by those skilled in the art based upon a review ofthe foregoing description of preferred embodiments.

We claim:
 1. A method of increasing the methane recovery from asubterranean carbonaceous formation penetrated by at least one well, themethod comprising:a) injecting a gaseous oxidant into the carbonaceousformation; b) maintaining at least a portion of the gaseous oxidant inthe carbonaceous formation for a selected time to stimulate theformation of cleats in the carbonaceous formation; and c) producingmethane from the carbonaceous formation at an increased rate.
 2. Themethod of claim 1 wherein the gaseous oxidant comprises a gaseousoxidant selected from the group consisting of ozone, oxygen andcombinations thereof.
 3. The method of claim 2 wherein the gaseousoxidant comprises ozone.
 4. The method of claim 3 wherein the ozone isdiluted with an inert gaseous diluent to form a gaseous oxidant mixturecontaining up to about 100 volume percent ozone.
 5. The method of claim2 wherein the gaseous oxidant comprises oxygen.
 6. The method of claim 5wherein the oxygen is diluted with an inert gaseous diluent to form agaseous oxidant mixture containing from about 23 to about 35 volumepercent oxygen.
 7. The method of claim 5 wherein the gaseous oxidant isair.
 8. The method of claim 5 wherein the gaseous oxidant isoxygen-enriched air.
 9. The method of claim 8 wherein theoxygen-enriched air contains at least about 30 volume percent oxygen.10. The method of claim 8 wherein oxygen-enriched air contains at leastabout 50 volume percent oxygen.
 11. The method of claim 1 wherein thegaseous oxidant is injected into the carbonaceous formation through awell; the well is shut-in for a selected time; and thereafter, methaneis produced from the well at an increased rate.
 12. The method of claim1 wherein the carbonaceous formation is a formation containingcarbonaceous materials which are present with inorganic materials in theformation.
 13. A method for increasing the gas permeability of asubterranean carbonaceous formation penetrated by at least one injectionwell and at least one production well, the method comprising:a)injecting a gaseous oxidant into the carbonaceous formation through theinjection well; b) maintaining the gaseous oxidant in the carbonaceousformation for a selected time to stimulate the formation of cleats andan increase in the surface area in the carbonaceous materials in theformation; and c) producing methane from the carbonaceous formationthrough the production well at an increased rate.
 14. The method ofclaim 13 wherein the gaseous oxidant comprises an oxidant selected fromthe group consisting of ozone, oxygen and combinations thereof.
 15. Themethod of claim 13 wherein the gaseous oxidant comprises ozone.
 16. Themethod of claim 15 wherein the ozone is diluted with an inert gaseousdiluent to form a gaseous oxidant mixture containing up to about 100volume percent ozone.
 17. The method of claim 13 wherein the gaseousoxidant comprises oxygen.
 18. The method of claim 17 wherein the oxygenis diluted with an inert gaseous diluent to form a gaseous oxidantmixture containing from about 23 to about 35 volume percent oxygen. 19.The method of claim 13 wherein the gaseous oxidant is oxygen-enrichedair.
 20. The method of claim 13 wherein the carbonaceous formation is aformation containing carbonaceous materials which are present withinorganic materials in the formation.